Chromium aldonate compositions and method for the preparation thereof



Patented Oct. 7, 1947 CHROMIUM ALDONATE COIVWOSITIONS AND METHOD FOR THE PREPARA- TION THEREOF Allan E. Chester, Highland Park, and Frederick F. Reisinger, Waukegan, 111., assignors to Poor & Company, Chicago, 111., a corporation Delaware N0 Drawing. Application March 20, 1944, Serial No. 527,359

14 Claims. (Cl. 260438) This invention relates to new and improved aldonate compositions, more particularly chromium gluconates, and a method for the preparation thereof.

One of the objects of the invention is to produce new and useful chromium aldonates.

A more specific object is the preparation of new and useful chromic gluconates.

A further object is the preparation of new organic complexes or compositions which remain in solution in cyanide-zinc electroplating baths and are effective in extending the range of current densities which may be used without burning the electroplated material.

Another object of the invention is to provide new and improved aldonate compositions which can be added to alkaline zinc cyanide electroplating baths and are effective in producing semibright, exceptionally smooth zinc plated articles having a pleasing uniform color, while using lower ratios of total sodium cyanide to zinc metal (M ratios) than has heretofore been considered possible for the production of brightening effects in the electrodeposition of zinc.

Another object of the invention is the provision of a new and improved method for producing compositions of the type described above. Other objects will appear hereinafter.

In accordance with the invention it has been found that by reacting together chromic compounds and aldonic acids new and improved aldonate compositions are obtained. These compositions exist in the form of aqueous solutions which have a green color varying from a deep green, almost black, shade in the concentrated form of the solution to a brilliant light green when the concentrated solutions are diluted.

The chromic aldonates prepared in the manner described apparently do not form chromium ions insolution as evidenced by the fact that the chromium will not plate out of these solutions. These solutions, moreover, exhibit excellent stability and do not tend to form precipitates even on standing over long periods of time.

The primary importance of the compositions prepared in accordance with the invention is in connection with the electrodeposition of metals, particularly in the electrodeposition of zinc from alkaline cyanide electrolytes. It has been found that when these solutions are added to cyanidezinc plating baths they are remarkably efiective as anti-burn agents and make it possible to deposit zinc over a wide range of current densities. Furthermore, it is possible by employing compositions of the type herein described in alkaline zinc cyanide plating baths, to obtain semi-bright, very smooth zinc plated finishes having a pleasing uniform color, with lower M ratios (ratio of total sodium cyanide to zinc metal) than have heretofore been considered possible in the production of brightening effects. Additionally, the aldonate solutions prepared in accordance with the invention, when added to cyanide-zinc plating baths, enhance the throwing and covering power of the baths. Although these compositions are primarily useful for electroplating operations, it will be understood that they can also be used for other purposes, for example, as water colors, mordants, and in other types of coloring or dyeing operations.

In the practice of the invention, a preferred type of product is a chromium gluconate which is preferably produced by dissolving chromic anhydride (CrOa) in Water, adding an alkali and then adding gluconic acid. A rather violent reaction occurs upon addition of the gluconic acid. Alternatively, the reaction may be carried out with a dichromate and gluconic acid in the presence of water.

The invention will be further illustrated, but is not limited, .by the following examples:

Example I An aqueous solution is prepared by dissolving 1100 grams of chromic anhydride (CrOs) in 1650 cc. of water. 900 grams of sodium hydroxide are then dissolved in this solution which becomes quite warm. This forms a solution of sodium chromate (NazCrOl). Thereafter, 6000 cc. of gluconic acid (containing about 1 to 4% dextrose or other reducing sugars) are added with agitation. In 4 or 5 minutes the resultant solution becomes very hot, with foaming and boiling and. the liberation of carbon dioxide. At the end of 1 to 2 hours the foam subsides, the product obtained being a slightly alkaline, very deep green solution having a specific gravity of about 1.3. This solution exhibits no precipitation or co-agulation on standing, and may be diluted with water to a beautiful green colored solution without precipitation. The chromium present does not plate out of the solution. The liberation of carbon dioxide is apparently due to the decomposition of organic substances, such as the reducing sugars which are oxidized. At the same time, it is quite apparent that chromium present in hexavalent form is reduced to its trivalent form.

Example II A solution was prepared as in Example I, ex-

cept that 1640 grams of sodium dichromate (Na2Cr2Om2H2O) were used instead of the chromic anhydride and sodium hydroxide. The sodium dichromate was dissolved in 1650 cc. of water and reacted with 6000 cc. of the 50% gluconic acid (as in Example I) with agitation. A green colored product was obtained as in Example I, having similar properties.

Example III This example illustrates the application of the products of Examples I and II in the preparation of alkaline zinc cyanide electrolytes.- An electrolyte was prepared by mixing together the following ingredients:

95-100 grams sodium cyanide;

45-55 grams zinc metal (added in the form of zinc oxide);

100-120 grams sodium hydroxide and enough water to make 1 liter of solution.

'2 grams per liter of zinc dust was stirred .into the electrolyte, and the entire mixture was filtered in order to remove traces of heavy metals. This electroplating bath, when employed in a conventional manner, ordinarily produces-zinc electroplated articles with a dull gray finish. Upon the addition of either of the products of Examples I or II, however, preferably in amounts corresponding to from 30 to 60 cc. of either of said products per gallon of the above described alkaline zinc cyanide electrolyte, semi-bright, smooth finishes, having a pleasing, uniform color, were obtained. Furthermore, it was found that the complex aldonate compositions of Examples I or II had a remarkable anti-burn efiect, and made it possible to obtain uniform, light gray colored zinc plates over a wide current density range. These compositions, therefore, are primarily important as range extenders or antiburn agents for electroplating baths. At the same time, however, they produce a noticeable effect on color and a substantial effect on the smoothness of the deposit.

One of the important features of the electrolytes containing compositions of the type herein described is the discovery that with such electrolytes substantial brightening efi'ects are obtained with M ratios within the range of 1.75 to 2.2. The expression M ratios, as previously indicated, is the ratio of the total weight of sodium cyanide in the bath to the total weight of Zinc metal. Heretofore it has been considered that an M ratio of 2.25 to 1 was the lowest M ratio at which bright zinc plates could be obtained. 8% Transactions of American Electrochemical Society, volume 80, page 416 (1941). As is well known, these higher M ratios decrease the cathode efficiency. However, by the incorporation into a cyanide-zinc electroplating bath of an aldonate of the type herein described an M ratio as low as 1.75 to 1 can be used with an increase in the cathode efiiciency to 94-96%. At the same time, high current densitiesup to 120 amperes per square foot, which is about all that is practical with present commercial installations, can be employed without burning the zinc plated article.

By using alternating currents superimposed upon ble to plate 0.0001 of zinc per minute with a 4 plating bath of the type described in Example III.

The invention is susceptible to some variation and momfication in the manner of its practical application. In a similar manner, other alkali metal hydroxides can be used in place of sodium hydroxides in Example I, including potassium hydroxide and lithium hydroxide. Alternatively, potassium dichromate can be used in place of the sodium dichromate in Example II.

In general, it is preferable to employ gluconic acid because it is the most cheaply and readily available of all of the aldonic acids. The gluconic acid is ordinarily employed in the form of an aqueous solution, and usually has a concentration not substantially greater than 50% be-' cause at the higher concentrations it tends to go into the crystalline lactone form. As examples of other aldonic acids which may be used, mention is made of the following: mannonic, arabonic, galactonic, and xylonic. These acids are obtained by the oxidation of the corresponding aldoses and all of them, including gluconic acid, have alpha and beta lactone forms.

The chromium compounds employed as starting'materials herein may be hexavalent and trivalent chromium compounds including, for example, ch'romic anhydride, sodium dichromate, sodium chromate, chromic hydroxide (Cr(Oh)3), potassium chromate and potassium dichromate. The compounds suitable for the practice of the invention are those which form solutions in water or in the presence of alkalies. It is preferable to have a substantial excess of alkali present during the reaction. Generally speaking, although the invention is not limited to the use of any specific proportions of materials, we prefer to use molecular ratios of chromium (Cr) to alkali met-a1 within the range of 1:1 to 1:3; molecular ratios of chromium to aldonic acid within the ratio of 1:1 to 1:3, preferably about 1.7; and a molecular ratio of total alkali metal plus aldonic acid to chromium at least 3:1 or greater.

The products of the invention may be described broadly as chromic aldonates and the products of Examples I and II as chromic gluconates. Where alkali metal ions are present during their formation the chromic aldonates will contain alkali metal atoms.

The invention is hereby claimed as follows:

1. The product of the reaction of an aqueous solution of a chromium compound selected from the group consisting of hexavalent and trivalent chromium compounds which form solutions in the presence of water and alkalies with an aldonic acid, the ratio of the aldonic acid molecule to the chromium atom being at least 1:1, said product existing in the form of clear green stable solutions in water.

2. The product of the reaction of an aqueous solution of a chromium compound selected from the group consisting of hexavalent and trivalent chromium compounds which form solutions in the presence of water and alkalies and gluconic acid, the ratio of the gluconic acid molecule to the chromium atom being at least 1:1, said product forming clear green stable solutions ,in water.

3. The product of the reaction of an alkaline aqueous solution of a ch'romate and an aldonic acid containing reducing sugars, the ratio of the aldonic acid molecule to the chromium atom being at least 1:1, said product existing in the form of clear green stable aqueous solutions.

4. The product ofthe reaction of an alkaline aqueous solution of a chromate and gluconic acid,

the ratio of the gluconic acid molecule to the chromium atom being at least 1:1.

5. An aqueous solution comprising the reaction product of a water soluble dischromate and an aldonic acid, the ratio of the aldonic acid. molecule to the chromium atom being at least 1:1.

6. An aqueous solution comprising the reaction product of sodium dichromate and an aldonic acid, the ratio of the aldonic acid molecule to the chromium atom being at least 1:1.

7. An aqueous solution comprising the reaction product of sodium dichromate and gluconic acid, the ratio of the gluconic acid molecule to the chromium atom being at least 1:1.

8. The method of preparing a chromic aldonate which comprises mixing together a solution of a chromium compound selected from the group consisting of hexavalent and trivalent chromium compounds which form solutions in the presence of water and alkalies and a solution of an aldonic acid, the ratio of the aldonic acid molecule to the chromium atom being at least 1:1.

9. The method of preparing a chromic gluconate which comprises mixing together a solution of an alkali metal dichromate, and a solution of gluconic acid containing reducing sugars, in the presence of sufficient water to permit boiling of the mixture due to its own heat of reaction, the ratio of the gluconic acid molecule to the chromium atom being at least 1:1.

10. The method of preparing a chromic aldonate which comprises reacting a water soluble chromic compound of an alkali metal with an aldonic acid in proportions such that the ratio the resultant solution approximately 1.? moles of gluconic acid for each mole of chromic anhydride.

12. The process of preparing a chromic gluconate which comprises reacting together an aqueous solution of sodium dichromate and an aqueous solution of gluconic acid, the ratio of the gluconic acid molecule to the chromium atom being at least 1:1.

13. The product obtained by reacting an aqueous solution of a chromium compound selected from the group consisting of hexavalent and trivalent chromium compounds which are soluble in the presence of water and alkalies with an aldonic acid, the ratio of the aldonic acid molecule to the chromium atom present in said chromic compound being at least 1:1, said product existing in the form of a clear stable solution in wa-- ter and being characterized by the fact that the chromium does not plate out of solution.

14. The product obtained by mixing together an aqueous solution of a chromium compound selected from the group consisting of hexavalent and trivalent chromium compounds which are soluble in the presence of water and alkalies and an aqueous solution of gluconic acid under conditions effective to cause an exothermic reaction to take place between said chromic compound and said gluconic acid, in the presence of an alkali metal compound, the ratio of chromium atom to alkali metal atom being within the range of 1:1 to 1:3, the ratio of chromium atom to gluconic acid molecule being within the range of 1:1 to 1:3, and the ratio of the total alkali metal atom plus the gluconic acid molecule to .the chromium atom being at least 3:1, said product forming clear green stable solutions in water.

ALLAN E. CHESTER. FREDERICK F. REIS-INGER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Beckers et al May 29, 1917 

